Thermoplastic Composite In-Situ Melt Processing Method for Composite Overwrapped Tools

ABSTRACT

An in-situ melt processing method for forming a fiber thermoplastic resin composite overwrapped workpiece, such as a composite overwrapped pressure vessel. Carbon fiber, or other types of fiber, are combined with a thermoplastic resin system. The selected fiber tow and the resin are prepared for impregnation of the tow by the resin. The resin is melted; and, carbon fiber is impregnated with the melted resin at the filament winding machine delivery head. The molten state of the composite is maintained and is applied, in the molten state, to the heated surface of a workpiece. The portion of the surface being wrapped is heated to the melting point of the thermoplastic resin so that the molten composite more efficiently adheres to the heated surface of the workpiece and so that the uppermost layer of fiber resin composite is molten when overwrapped resulting in better adherence of successive layers to one another.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Non-Provisional patentapplication Ser. No. 15/340,005, filed on Nov. 1, 2016, which claimedthe benefit of U.S. Provisional Patent Application No. 62/249,467, filedon Nov. 2, 2015, each of which is incorporated herein in its entirety byreference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

BACKGROUND OF THE INVENTION 1. Field of Invention

The invention relates to a method of overwrapping a fiber resincomposite over a tool that allows the optimized placement of individualfilaments of a fiber tow at the appropriate location such asoverwrapping a fiber matrix lamina over a tool. More particularly, itrelates to an in-situ melt process, (from dry fiber to molten laminaapplication), of combining a fiber and a resin and applying the moltenfiber resin combination onto a tool in one continuous operation in amanner that more efficiently uses each individual filament to obtainoptimized performance of the structure.

2. Description of the Related Art

In the field of carbon fiber and resin composites, due to the higherstrength to weight ratio of carbon fiber over steel, numerous tools andstructural components, such as concrete pilings, aircraft wings andfuselages, automotive applications, and sporting goods are increasinglysubstituting carbon fiber and resin composites for steel. CompositeOverwrapped Pressure Vessels, (“COPV”), are among the structures thatmake use of carbon fiber composite technology. And, it is known thatthere are a number of COPV designs in the market today. In this regard,Type II pressure vessels utilize hoop wraps of fiber over a metallic,usually steel, liner. Type III pressure vessels utilize both hoops andhelical wraps of fiber over a metallic liner, usually aluminum, liner.Type IV pressure vessels utilize hoops and helical wraps over a plasticliner. And, Type V pressure vessels utilize hoop and helical fiber wrapsover a liner-less tool that may incorporate a barrier film to preventgas permeation. While the present invention has utility within thesetypes of pressure vessels, as described herein, it also has utility inother types of tools that utilize a composite overwrap. The majority ofthese designs use a traditional wet winding process with carbon fiberand thermosetting resin. And, it is well known that the currentmaterials and processes used to manufacture tanks today are costly andlaborious.

The world-wide demand for fiber, including carbon fiber, for COPVs isgrowing. Fiber is used for tanks for CNG vehicles, pipelines, storage,and transportation for gases, including without limitation, CNG,Hydrogen, Nitrogen and other gases. Those skilled in the art expect thisdemand to continue to grow, driven mostly by the class 8 truck market,especially in the event that oil prices begin to rise again and therefueling infrastructure expands and matures.

As stated above, the main cost drivers for COPVs are material,manufacturing time, ultimate performances, and coefficient ofperformance variances. The current COPV manufacturing process results ina long process cycle for tank manufacturing and consists of thefollowing steps: one of several known methods of forming a fiber/resincomplex is selected; the fiber/resin complex is wound onto the pressurevessel body; the pressure vessel body is rotated until B-stage isachieved; the would cylinder is cured at an elevated temperature in anoven; and the cylinder is cleaned and packaged.

If COPVs are to be widely adopted into the high-volume markets, such asthe transportation market, a higher speed, lower cost, manufacturingprocess (elimination of equipment and process steps), increase inultimate performances, and decrease in the coefficient of performancevariances is necessary.

Carbon fiber is typically delivered as single individual fiber filamentsbundled up into a tow. Moreover, a tow may comprise as few as a thousandor as many as twenty-four thousand, or more, individual micron sizecarbon filaments. Depending upon the bandwidth of the tow as the fiberis being wrapped around a workpiece, the tow may be several hundredfibers thick. As used herein, the term “bandwidth” refers to the totalspread width of a fiber tow. And, it is known that it is desirable andcritical to maintain the individual filaments within a tow at aconsistent tension/length both from side to side and from top to bottomas the tow is wound around a workpiece such as a pressure vessel. Inthis regard, it is known that if each individual filament is fixed inrelation to each other, as with a rigid construction (plastic tape andtowpreg) is wound around a radius, it is difficult to maintainconsistent individual filament tension. Stated another way, as fiber ina towpreg that has been pre-impregnated with matrix resin and cooled ina straight, or potentially a curved position, then wrapped around acurved surface, the top filaments, i.e. the filaments on the exterior ofthe radius, are in tension and bottom filaments, i.e. the filamentsalong the interior of the radius, are compressed. This does not allowthe individual filaments to slide relative to one another causing aphenomenon of catenary, which results in the compression layers of thetowpreg embedding into the composite as wavy, wrinkled, or creasedfiber, which decreases the overall performance of the structure. This isbecause every individual filament does not contribute uniformly to theoverall composite performances.

When this is manufactured in this manner, the individual filaments arelocked into a certain position. That is, some individual filaments aresubjected to the structural load while others are not being utilized. Inthis regard, and referring to FIGS. 1-3, a wrapped vessel is illustratedschematically as vessel 300. Vessel 300, in this example, has a first,or inner, wrapped layer 310 and a second, or outer, wrapped layer 320.It will be understood that FIGS. 1-3 are not drawn to scale. Withinlayer 310 there is an outer region 312 in tension and an inner region315 in compression. Similarly, within layer 320, there is an outerregion 322 in tension and an inner region 325 in compression. At theinitial winding and at neutral pressure, P0 in FIG. 1 the filaments inthe tension regions 312 and 322 are fully aligned and are fully engagedwhile the filaments in the compression regions 315 and 325 are wavy,wrinkled, or creased. As pressure is added to the vessel, (pressurebeing indicated by arrows 350), designated P1 in FIG. 2, it will berecognized that the vessel undergoes a slight dimensional diameterchange. As a result, the filaments in the tension regions 312 and 322are still fully aligned with fibers fully contributing to compositestrength. However, the filaments in the compression regions 315 and 325,are wavy and/or creased and are not fully contributing to compositestrength. Finally, as greater pressure is exerted on the vessel 300, P2in FIG. 3, with additional diameter change, the fully aligned filamentsin the tension regions are now at their ultimate strain/stresscapability while the fibers in the compression regions are still notfully contributing to fully optimized composite strength such that allof the pressure load has transferred to the filaments in the tensionregions. The result is premature failure as the well-aligned filamentsin the tension regions fracture first. The majority of the load is thentransferred immediately to the non-fully aligned filaments in thecompression regions which immediately fracture as well. Lack ofuniformly distributed filament alignment causes several things to occur,such as non-uniform increase in dimensional changes, variability inperformances, and ultimate fiber performances are not achieved resultingin premature failure. This premature failure results in increaseddeformation and increased diameter of the vessel 300. Ultimate compositeperformance is not achieved because the full potential of all of thefilaments is not achieved such that the full potential of the filamentmodulus is not utilized. This problem is compounded with workpieces thathave a complex geometric shape.

While various known wet winding processes alleviate some of theseproblems to some extent because the individual filaments are allowed toslide relative to each other, known wet winding processes suffer fromother disadvantages including the necessity of using a low viscosityresins, prolonged curing times, and non-uniform bandwidth shape andsize. Further, with known thermoplastic resin systems, a hot molten tapelayer is applied to a cool solidified thermoplastic structure. Hence,one layer does not fully adhere to the previous layer, sometimesreferred as a cold flow front. This results in low interlaminar shearstrength and, ultimately, will prevent the composite from acting as onecontinuous composite structure leading to premature failure of thevessel.

What is missing from the art is an in-situ melt process of combining acarbon fiber with a thermoplastic and applying the molten composite to awork piece in a manner that allows most if not all of the filaments inthe wound composite to determine full composite stiffness (moduluspotential) and contribute to full composite strength performances. Whatis further missing from the art is an in-situ process that applies themolten thermoplastic composite to a portion of a tool that has beenheated to the melting point of the thermoplastic composite in such amanner that as successive layers of molten thermoplastic are applied anarea of previously applied thermoplastic resin is heated to the meltingpoint such that there is improved adhesion between successive layers,thereby effectively eliminating catenary, wrinkles and creases with thethermoplastic composite overwrap. Also, where successive layers haveimproved adhesion to one another, interlaminar shear strength isincreased. Accordingly, one of the objects of the present invention isto create a method that reduces the overall cost of a COPV through thedevelopment of a thermoplastic design and a unique manufacturingprocess. A further object of the present invention is develop athermoplastic system consisting of a unique rapid filament windingmanufacturing process to produce composite structures. Still anotherobject of the present invention is to use a selected fiber with athermoplastic resin system using an in-situ process (combine both fiberand thermoplastic resin at the filament winding machine delivery head,an automatic tape laying head, or a tow laying system). Such an in-situprocess, (combination of a thermoplastic film and carbon fiber at ornear the delivery head to allow molten composite be applied to the toolsurface), would reduce production time and cost. It will be recognizedthat reference to a “delivery head” is inclusive of filament windingmachines, automatic tape laying systems, tow laying systems, and othersystems, presently known or to be developed, for applying a moltenthermoplastic composite to a tool.

BRIEF SUMMARY OF THE INVENTION

The present invention is directed towards a method of creating a fiberresin composite overwrap for various workpieces that are known to beamenable to the composite overwrap process. While the present inventionhas utility in many types of applications that use composite overwrap, aprimary usage is in the preparation of Type II, Type III, Type IV andType V pressure vessels, referred to herein as composite overwrappedpressure vessels, or COPV's. Type III COPV's tend to be modulus drivenapplications while Type IV COPV's tend to be strength drivenapplications. Those skilled in the art recognize that strength drivenapplications call for fibers having different properties than fibersselected for modulus driven applications. The various physicalattributes of various fibers are within the scope and spirit of thepresent invention. Carbon fiber, or other types of fiber, are combinedwith a compatible thermoplastic resin system. The selected fiber,whether carbon, glass, aramid, natural fiber, nano-fiber, or other knownfibers are prepared for impregnation by a thermoplastic resin. Theselected thermoplastic resin, whether in a pellet, tape, or threadconfiguration, is also prepared for processing.

The carbon fiber and reduced viscosity, i.e. melted, thermoplasticresin, are combined at the filament winding machine delivery head, underpressure, thereby forcing the resin into the fiber bundle. The moltenfiber resin lamina is maintained until it is applied to a localizedheated portion of the surface of a workpiece, such as a pressure vessel.

The portion of the surface of the workpiece that is to be wrapped isheated to the melting point of the thermoplastic resin so that themolten composite more efficiently adheres to the heated surface of theworkpiece. Further, as additional layers of molten thermoplasticcomposite are wrapped, at least the portion of previously wrappedthermoplastic resin composite is reheated and remelted such that thelayers of composite remain molten during the over-wrapping processresulting in better adherence of the layers to one another. The moltenlayers are then compacted and consolidated under pressure forcing thetwo distinct resin layers to mix and mingle (allowing polymer chains tocross the boundary) which provides a uniform homogeneous structure. Thiscompaction process removes entrapped air and consolidates the variouslayers of overwrapped fiber resin composite. Use of an in-situ meltprocess increases composite performance in that catenary, creases, andwrinkles are diminished or eliminated. This is because the individualfilaments throughout the thickness of the molten fiber resin compositetowpreg bandwidth are allowed to slide across each other during thewrapping process thereby allowing for uniform tension within the fiberbundle of the fiber resin composite.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The above-mentioned features of the invention will become more clearlyunderstood from the following detailed description of the invention readtogether with the drawings in which:

FIGS. 1-3 are schematic views of prior art wrapped pressure vesselsschematically depicting differing pressures being exerted on saidvessels;

FIGS. 4A-4E depict the matrix of choices available at each processingstep in the method of the present invention;

FIGS. 5A, 5B and 5C depict a flow-chart of the steps of the presentinvention;

FIG. 6 is a schematic view of an exemplary embodiment of the invention;and

FIG. 7 is an enlarged schematic view of a portion of FIG. 6 illustratingthe mixing of the molten and compacted layers of resin fiber composite.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed towards a method of creating a carbonfiber overwrap, such as is used for pressure vessels. Those skilled inthe art will recognize that other types of items, referred to hereingenerally as workpieces, are often overwrapped with a fiber resincomposite. The present invention is further directed towards using anin-situ process for combining both the fiber and thermoplastic resin atthe filament winding machine delivery head, the delivery head of anautomatic tape laying system, or the delivery head of a fiber resincomposite towpreg laying system, and applying the molten fiberthermoplastic resin complex to a heated workpiece, such as, but notlimited to a pressure vessel. As used herein, the term “in-situ process”refers to a continuous process, from impregnation of the dry fiber tothe final application, for allowing optimized filament placement. Thein-situ process of the present invention allows the individual filamentsto slip, slide, or shear relative to one another, from before theimpregnation process until final placement onto a tool/compositestructure whereby any change in length from side to side and/or from topto bottom, each and every individual filament dimension is translatedall the way back to the original dry fiber.

Raw Materials

Those skilled in the art will readily recognize that the primary rawmaterials in a carbon fiber resin composite include the resin 10 and thecarbon fiber 20. The discussion to follow will address these componentsin greater detail and their relationship to one another is seen both inFIGS. 4A-4E which depicts a matrix of choices at each processing stepand in FIGS. 5A, 5B and 5C which depict a flow-chart of the steps of thepresent invention.

Carbon Fiber

Referring to FIG. 4A and the step 20 of selecting a fiber, it will berecognized that there are a number of commercially available fibers thatare used with composite overwrapped applications. In this regard, carbonfibers 22 are typically chosen for Type IV COPV's that are typicallyused with compressed natural gas, (“CNG”), due to their strength. TypeIV COPV's used with CNG are a strength driven application for compositeoverwrapping due to the fact that the plastic liner does not fatigue.Other applications may call for use of glass fiber 21, natural fiber,nano-fiber, or aramid fibers 23. And, those skilled in the art willrecognize that there are other 24 known types of fibers that arecompatible with thermoplastic resin systems. Type III COPV's used forCNG are modulus driven applications because of the known tendency ofaluminum liners to fatigue. Those skilled in the art recognize thatstrength driven applications call for fibers having different propertiesthan fibers selected for modulus driven applications. The variousphysical attributes of various fibers are within the scope and spirit ofthe present invention.

It is known in the art that various commercially available fibersconsist of sizing, which is not compatible with thermoplastic resinsystems. Those skilled in the art recognize that sizing is a protectivefilm used to protect the individual fiber filament and allow subsequenthandling of the fibers; and that sizing promotes the adhesion betweenthe fiber and matrix. Accordingly, in an exemplary embodiment, a fiberhaving sizing chemistries compatible with thermoplastic resins, such aspolypropylene and nylon resins is selected. In an exemplary embodiment,the sizing is applied to the fiber during the carbon fiber manufacturingprocess, not after. Additionally, due to the known tendency of somefibers and some sizing chemistries to absorb moisture, any latentmoisture must be removed by drying. Otherwise the moisture may beexpelled during the process of combining the fiber and the resinresulting in porosity within the structure. This porosity results inpoor product performance quality. In an exemplary embodiment, the fibercan be dried in oven dryers or infrared dryers. Those skilled in the artwill recognize that there are other known methods of drying fiber.

Resin

Referring to FIG. 4A and the step 10 of selecting a resin, and inaccordance with the present invention, thermoplastic resins, as opposedto epoxy resins, are preferred. There are many commercially availablethermoplastic resins 11, including nylon resins 12, polypropylene resins13, polyethylene resins, and polyetheretherketone, (“PEEK”), resinsystems. Those skilled in the art will recognize that there are other 14commercially known thermoplastic resin systems that could easily beadapted to the in-situ process of the present invention. For Type IVpressure vessels the use of a low temperature thermoplastic is requireddue to the plastic liner that is being used. In an exemplary embodiment,polypropylene resin is utilized with plastic liner Type IV pressurevessels. In a further exemplary embodiment, for Type III pressurevessels, a higher temp resin system, such as a nylon resin system isselected.

Referring to FIG. 4B, just as thermoplastic resins are available in avariety of different chemistries, these resins are available in avariety of physical forms. Thermoplastic resins are available in aspellets 30, films 40, and threads 50. While each of these can be adaptedto be used in the present in-situ melt process, it is more economical toutilize a resin in a pellet form. As described above with fiber, it isimportant that any absorbed moisture be driven off by drying process 31.In this regard, Hooper dryers 32, oven dryers 33. Additionally, otherdrying systems 34, such as conduction rollers, and infrared dryers canbe utilized to dry the thermoplastic resin. Moisture can also beremoved, as will be recognized by those skilled in the art, by the useof oven dryer systems. Similarly, as set forth in FIG. 4B, when usingfilms 40 absorbed moisture should be driven off by a drying process 41,which could, within the scope of the present invention, include ovendryers 42, infrared dryers 43, conduction rollers 44, or other 45 dryingmethods. Still referring to FIG. 4B, when using thread resins 50,absorbed moisture should be driven off by a drying process 51, whichcould, within the scope of the present invention, include oven dryers52, infrared dryers 53, other 54 drying methods. Further, as seen inFIG. 4B, when using fiber resins 60, absorbed moisture should be drivenoff by a drying process 61, which could, within the scope of the presentinvention, include oven dryers 62, infrared dryers 63, other 64 dryingmethods.

Material Preparation

Referring to FIG. 4C, subsequent to drying the thermoplastic resin, itis necessary to significantly reduce the viscosity 70 of the resinmaterial. The resin's minimum viscosity is reduced under pressure, at aselected temperature over a selected period of time. In this regard,time, temperature, and pressure are inter-dependent variables in theprocess of reducing the resin to its minimum viscosity. In an exemplaryembodiment, shearing arbors 74 are used to decrease the viscosity of theresin by increasing friction, pressure, and time. Heat is applied by useof conduction heaters 71, induction heaters, infrared heaters 73, orother 75 presently known, or subsequently discovered methods of heatinga thermoplastic resin in order to reduce its viscosity. It will beunderstood that the desired temperature is dependent, first, on theresin chemistry selected. Once the viscosity is reduced to a desiredlevel, the melted resin is combined with the fibers, which have beenfurther prepared as will be described below, under pressure in order toassure that the melted resin completely infiltrates the fiber tow.

Prior to being infiltrated by the melted resin, the fiber must beprepared 80. In order to provide optimum performance, the load-bearingfiber must be uniformly and equally spread to the desired bandwidth.Further, during the spreading process and during the infiltration step,care must be taken to ensure that every individual filament within thecarbon fiber tow is under the same tension. This critical step alsoallows easy resin infiltration into and around the individual filaments.In an exemplary embodiment, upstream tension and downstream tension areisolated from one another 81. Tension up-stream of the impregnation areais kept at a minimum, to prevent fiber damage. Pressure is also kept ata minimum during the impregnation process to allow easy infiltration ofresin. However, after the impregnation process, pressure is increased toimprove individual filament alignment and uniformity. The raw fiber towis also spread 85 to the desired bandwidth 83 and tension. Mechanicalrollers 86, air flow 87, ultrasonic devices 88, and other devices 89,such as combs, are known devices for spreading the raw fiber tow to thedesired bandwidth and maintaining tension on the fiber tow such that allfibers in the fiber tow are under the same tension.

After the individual filaments of the fiber tow are uniformly andprecisely spread 85 to the desired bandwidth, the fiber tow must beheated 90 to the same, or in some instances higher, temperature as themolten resin in order to assure adequate and complete impregnation.Those skilled in the art will recognize that the fiber tow can be heatedby use of an oven 91, an induction furnace 92, or through the use ofother methods 93, such as conduction rollers.

Material Combination—Resin Impregnation of Fiber

As described above, after the fiber tow is prepared, i.e. dried, spreadto the desired bandwidth, and heated, and the resin is at its minimumviscosity, the fiber tow is impregnated, i.e. combined and consolidated100, with the molten resin in a selected, determined amount 101, underpressure in an exemplary embodiment. Impregnation under pressure, eitherpositive pressure or negative pressure, can be accomplished by acompaction press 102, a compaction roller 103, a compaction die 104, bypressure impregnation 105, or by other methods 106. In an exemplaryembodiment, this step is accomplished immediately prior to applicationof the molten composite to a heated area of a workpiece such as pressurevessel. In this regard, in the in-situ process of the present invention,the fiber tow is impregnated with molten resin at, or in very closeproximity to the filament winding head.

Material Application—Carbon Fiber In-Situ Process

Once the fiber and resin are combined, under pressure, to form a moltenfiber resin composite towpreg, the molten fiber resin composite towpregis kept at a constant bandwidth, tension, and temperature 110. Asdiscussed above, tension is controlled, and upstream tension is isolatedfrom downstream tension 81. Bandwidth and temperature are controlled asdiscussed above. The location 111 where the molten fiber resin compositetowpreg will be laid down is brought to and kept at a temperature thatis approximately the same as the melting point of the thermoplasticresin system in order to allow adhesion of one layer to another,previously wrapped layer. A heat source 112, such as an infrared source,induction coils, conduction rollers, flames, infrared heaters, by way ofexample and not limitation, is utilized to This allows each individualfilament to slide one relative to another thereby allowing eachindividual filament to be at the relative same tension duringapplication of the molten fiber resin composite towpreg. Further,maintaining a molten state as the layers are laid down 113 allowsentrapped air to escape. A heat source 114, such as an infrared source,induction coils, conduction rollers, flames, infrared heaters, by way ofexample and not limitation, is utilized to heat the pressure vessel orother tool being overwrapped. An external compaction force 115 isutilized to compact and consolidate the molten fiber resin compositetowpreg to the pressure vessel, or other tool being overwrapped. Thiscompaction process removes entrapped air 116 and consolidates thevarious layers of overwrapped fiber resin composite. A compactionroller, nip roller 117, or similar device 118, is utilized for thiscompaction process.

It will be understood by those skilled in the art, that the entirepressure vessel could be brought up to temperature, or, in an exemplaryembodiment, the heating could be isolated, or localized, to the portionof the tool surface undergoing the overwrap process. Further, with smallvessels or tools, the temperature is maintained at the melting point ofthe thermoplastic resin as successive laminations 113 of fiber resincomposite are laid down.

In an exemplary embodiment, and referring to FIGS. 5A and 5B, resin isselected 10 and the fiber is selected 20. In an exemplary embodiment,the selected fiber is a carbon fiber. After the fiber is conditioned 60and prepared 80, and after the viscosity of the selected resin isreduced 70, the carbon fiber and the thermoplastic resin are combined100 at the filament winding machine to produce a thermoplastic compositethat is then delivered to a workpiece, such as a pressure vessel, whilestill molten. Those skilled in the art will recognize that othercomposite manufacturing equipment, such as automatic tape laying systemsand other tow laying systems could be utilized with the process of thepresent invention. To achieve this objective, as stated above, the fiberand resin are heated and combined 100 under pressure in order toimpregnate the carbon fiber with molten thermoplastic resin. Then whileit is molten, the molten composite matrix is applied to the surface ofthe tool, (pressure vessel liner, shaft or other structure), which hasalso been heated to the melting point of the thermoplastic resin. Inthis regard, in an exemplary embodiment, a heating system, such as aflame or heated conduction roller system, is utilized to heat at leastthe portion of the surface of the tool to which the molten fiber resincomposite towpreg is to be applied/wrapped. Those skilled in the artwill recognize that other heat sources could be utilized. Carbon fibersquickly absorb this heat (Induction heat) and will automaticallytransfer this heat to the thermoplastic resin.

The molten fiber resin composite lamina could then be compressed, bymeans of compaction rollers 180, 185, dies, or other devices into tapeform of the desired bandwidth. This hot composite, i.e. molten fiberresin composite towpreg, which in one embodiment could take the form ofa molten fiber resin tape, is then applied to a hot tool surface. Bymaintaining at least a selected portion of the working surface of theworkpiece 200 at an elevated temperature 111 that approximates themelting point of the thermoplastic resin and by compacting successivemolten lamina under pressure, successive layers adhere to one anotherand fuse thereby eliminating, or substantially eliminating cold flowlines, sometimes referred to as knit lines. Thus, in the in-situ meltprocess of the present invention, at least a selected area of theworkpiece, tool or composite is heated prior to consolidation throughthe use of another heat source, which can be gas, inductive, infrared,or other known heat sources.

In an exemplary embodiment, immediately prior to the step of wrapping,or applying, the first layer of molten fiber resin composite towpreg toa workpiece 200, at least a selected portion of the workpiece 200 isheated 120 to the melting point of the molten fiber resin compositetowpreg such that the molten state of the molten fiber resin compositetowpreg is maintained during the wrapping process. The first layer ofmolten fiber resin composite towpreg is applied to the heated surface ofthe workpiece 125. Applying the molten fiber resin composite towpreg tothe heated surface of the workpiece enhances adhesion and also allowsthe layers of fiber within the molten fiber resin composite towpreg toslide relative to one another maintaining uniform tension andsubstantially eliminating catenary and wrinkling.

Thereafter, as additional layers of molten fiber resin composite towpregare applied, at least a selected portion of the outermost layer of thewrapped workpiece is heated 130 such that the previously applied,outermost layer of the fiber resin composite towpreg is melted prior toapplying, or over-wrapping, an additional layer of molten fiber resincomposite towpreg. A subsequent, or next, layer of molten fibercomposite towpreg is then wrapped over the melted substrate layer 135.Melting the substrate layer, i.e. the previously wrapped layer of fiberresin composite towpreg prior to applying the next layer of molten fiberresin composite towpreg results in applying molten fiber resin compositetowpreg to a molten substrate. This results in better adherence ofsuccessive molten composite layers to one another and further allowsindividual filaments within said molten fiber resin composite towpreg toslide relative to one another, again, thereby effectively eliminatingcatenary, wrinkles, and creases within the fiber resin compositeoverwrap. Further, as the molten fiber resin composite towpreg isapplied to the molten substrate fiber composite, the successive layersare compacted and consolidated 115 under pressure thereby removingentrapped air. This process continues 140 until the process is complete.

This process is also represented schematically in FIGS. 6 and 7 in whichthe selected, and prepared fiber bundle/tow 145 is impregnated withmolten resin at the impregnation tool 150. Fiber bundle/tow 145 iscomprised of potentially thousands of individual filaments that actindependently of each other. That is, each individual filament has theability to slide, slip or shear relative to one another. Theseindividual filaments depending on their length determine the overallperformances of a composite structure. When some of these individualfilaments are longer, they initially will not contribute to themechanical performance of the composite structure.

In this regard, impregnation tool 150 introduces a matrix/resin into thefiber bundle/tow 145. In this particular embodiment, it introduces amolten resin system into the fiber bundle/tow 145. To assure properperformance, each individual filament is coated with this resin system.In this molten state the individual filaments have the ability to slide,slip or shear relative to one another. In the in-situ process of thepresent invention, the fiber tow is impregnated with molten resin at, orin very close proximity to the filament winding head. Further, once thefiber and resin are combined, under pressure, to form a molten fiberresin composite towpreg 155, the molten fiber resin composite towpreg iskept at a constant bandwidth and tension and the temperature ismaintained at the melting point. As discussed above, tension iscontrolled and upstream tension 160 is isolated from downstream tension165. Bandwidth and temperature are controlled as discussed above. Oncethe molten lamina formed by fiber resin composite towpreg 155 leaves theimpregnation tool 150, it remains molten. Any path that this moltenlamina takes over any heated structure or any other heated support toolallows the individual filaments to slide, slip or shear relative to oneanother. Any individual filament movement (slide, slip or shear) istransferred and related back to the original fiber bundle 145. Forfilaments that have low elongation properties this is particularly true.

As the molten lamina fiber resin composite towpreg 155 enters intoheated sections 160 and 165, it is kept in a molten state to allow eachindividual filament within the tow to slide, slip, and shear relative toone another. In the heated sections 160 and 165, there can be a seriesof equipment that maintains the molten state. It can be comprised ofrollers, heaters (infra-red, convection and or lasers) that maintainthis molten state.

As stated above, at least the portion of the workpiece 200 that is to bewrapped is heated 120 to the melting point of the molten fiber resincomposite towpreg 155 such that the molten state of the fiber resincomposite towpreg is maintained during the wrapping process. In anexemplary embodiment, this is accomplished by a heated compaction roller185. Heated compaction roller 185 heats the portion of the workpiece 200to be wrapped, or as the wrapping process continues the portion of theoutermost layer of fiber resin composite that is to be over-wrapped tothe melting point so that the outermost layer of fiber resin compositeand the molten fiber resin composite towpreg are both at the meltingpoint of the fiber resin composite. A second heated compaction roller180 compacts and consolidates the molten layers under pressure therebyremoving entrapped air.

On the localized composite structure 190 the surface on this structureis heated to the molten state by a heating system 185. This compositematrix surface is now translated to position (moved by rotation or othermeans) 195 where the molten lamina fiber resin composite towpreg 155 isapplied to this surface. Both surfaces 195 and 155 are at a moltenstate. A localized heated compaction system 180 applies temperature andpressures allowing both molten matrixes to mix and adhere to oneanother. As the molten lamina fiber resin composite towpreg 155 isapplied to localized molten surface 195 any change in geometric surfacepath that changes fiber length, this change in length is transmitted allthe way back to the original dry fiber 145. Because the filament laminafiber resin composite towpreg 155 is molten each individual filamentacts independently of one another. Hence each individual filament isallowed to slide slip or shear relative to one another.

At location 205 majority of the individual filaments within this sectionof the composite structure are at, straight uniform and consistent. Thatis each individual filament at a uniform and consistent tension. Whenthis structure is mechanically loaded majority of individual filamentare contributing to the performance of the composite. Hence, theperformance of the structure is consistent and uniform.

At location 205 the matrix resin system throughout the thickness of thestructure is uniform and consistent. It will be appreciated that duringthe wrapping process, as the molten lamina fiber resin composite towpreg155 that has been applied to workpiece 200 passes location 205, fiberresin composite towpreg 155 is allowed to cool below the melting pointof the resin. When a molten lamina comprised fiber and matrix/resin isapplied to the surface of 195 there exists a distinct easily observableboundary layer. The matrix/resin polymer chain 215 in area 195 and inthe molten lamina fiber resin composite towpreg 155 are segregated,polymer chains do not cross this boundary. Once additional heat andcompaction 180 is applied during the wrapping process, these boundarylayers fade and dissolve as the matrix/resin layers are compacted andare thereby forced to mix. Now across this boundary layer the polymerchains do mingle and strengthen and behave as a uniform structure.

Use of an in-situ melt process also increases composite performance inthat catenary, creases, and wrinkles are diminished or eliminated. Inthis regard, each individual filament within the carbon fiber resin towprepreg is allowed to be at equal and uniform tension. This is achievedbecause the resin system in the towpreg is allowed to be moltenthroughout the process thereby allowing each individual filament withinthe molten fiber resin composite towpreg to slide relative to oneanother, thereby achieving full potential of the molten fiber resincomposite towpreg's mechanical properties and thereby increasinginterlaminar shear strength. This is because the inner and outer layersof carbon fiber within the molten fiber resin composite towpregcomposite are allowed to slide across each other allowing for uniformtension within the fiber bundle of the fiber resin composite. Further,it will be recognized that the thermoplastic in-situ melt processdescribed herein can be used in any composite manufacturing equipment.That is it can be used within a fiber placement machine or otherequipment such as a pultrusion system and similar machinery.

While the present invention has been illustrated by description ofseveral embodiments and while the illustrative embodiments have beendescribed in detail, it is not the intention of the applicant torestrict or in any way limit the scope of the appended claims to suchdetail. Additional modifications will readily appear to those skilled inthe art. The invention in its broader aspects is therefore not limitedto the specific details, representative apparatus and methods, andillustrative examples shown and described. Accordingly, departures maybe made from such details without departing from the spirit or scope ofapplicant's general inventive concept.

Having thus described the aforementioned invention, what is claimed is:1. An in-situ melt process for creating a fiber resin compositeoverwrap, said in-situ melt process comprising the steps of: selecting afiber tow; selecting a thermoplastic resin compatible with said selectedfiber tow; preparing said fiber tow for impregnation by said selectedthermoplastic resin; preparing said thermoplastic resin for impregnationinto said fiber tow; reducing viscosity of said thermoplastic resin byheating said thermoplastic resin to a melting point of saidthermoplastic resin; impregnating said prepared molten thermoplasticresin into said prepared fiber tow in close proximity to a filamentwinding head, thereby creating a molten fiber resin composite towpreg;heating a selected portion of a workpiece to said melting point of saidthermoplastic resin; applying a first layer of said molten fiber resincomposite towpreg to said heated surface of said workpiece whereby amolten state of said molten fiber resin composite towpreg is maintainedduring a wrapping procedure thereby resulting in said molten fiber resincomposite towpreg more efficiently adhering to the heated surface of theworkpiece; heating a selected portion of said workpiece having anapplied layer of said molten fiber resin composite towpreg, wherein saidapplied layer of said molten fiber resin composite towpreg is remeltedprior to applying an additional layer of said molten fiber resincomposite towpreg; applying an additional layer of said molten fiberresin composite towpreg to said molten applied layer of fiber resincomposite towpreg whereby applying molten fiber resin composite towpregto an area of molten fiber resin composite towpreg previously applied tosaid workpiece allows successive molten fiber resin composite layers toadhere to one another resulting in greater interlaminar shear strengthand further allows individual filaments within said molten towpreg toslide relative to one another, thereby effectively eliminating catenary,wrinkles, and creases within the fiber resin composite overwrap; andcompacting and consolidating said molten fiber towpreg layers to oneanother under pressure thereby removing entrapped air.
 2. The in-situmelt process for creating a fiber resin composite overwrap of claim 1wherein said workpiece is a Type II pressure vessel.
 3. The in-situ meltprocess for creating a fiber resin composite overwrap of claim 1 whereinsaid workpiece is a Type III pressure vessel.
 4. The in-situ meltprocess for creating a fiber resin composite overwrap of claim 1 whereinsaid workpiece is a Type IV pressure vessel.
 5. The in-situ melt processfor creating a fiber resin composite overwrap of claim 1 wherein saidworkpiece is a Type V pressure vessel.
 6. The in-situ melt process forcreating a fiber resin composite overwrap of claim 1 wherein said fiberis selected from a group consisting of carbon fiber, glass fiber,natural fiber, nano-fiber, and aramid fiber.
 7. The in-situ melt processfor creating a fiber resin composite overwrap of claim 1 wherein saidthermoplastic resin is selected from a group consisting of nylon resin,polypropylene resin, polyethylene resin, and polyetheretherketone resin.8. The in-situ melt process for creating a fiber resin compositeoverwrap of claim 1 wherein said thermoplastic resin is in pellet form.9. The in-situ melt process for creating a fiber resin compositeoverwrap of claim 1 wherein said thermoplastic resin is in tape form.10. The in-situ melt process for creating a fiber resin compositeoverwrap of claim 1 wherein said thermoplastic resin is in thread form.11. The in-situ melt process for creating a fiber resin compositeoverwrap of claim 1 wherein the step of preparing said fiber tow forimpregnation by said thermoplastic resin includes the steps of dryingsaid fiber tow, spreading said fiber tow to a selected bandwidth, andheating said fiber tow to a selected temperature, wherein said selectedtemperature is approximately a melting point of said selectedthermoplastic resin.
 12. An in-situ melt process for creating a fiberresin composite overwrap, said in-situ melt process comprising the stepsof: selecting a fiber tow; selecting a thermoplastic resin compatiblewith said selected fiber tow; preparing said fiber tow for impregnationby said selected thermoplastic resin, wherein the step of preparing saidfiber tow for impregnation by said thermoplastic resin includes thesteps of drying said fiber tow, spreading said fiber tow to a selectedbandwidth, and heating said fiber tow to a selected temperature, whereinsaid selected temperature is approximately a melting point of saidselected thermoplastic resin; preparing said thermoplastic resin forimpregnation into said fiber tow; reducing viscosity of saidthermoplastic resin by heating said thermoplastic resin to a meltingpoint of said thermoplasitic resin; impregnating said prepared moltenthermoplastic resin into said prepared fiber tow in close proximity to afilament winding head, thereby creating a molten fiber resin compositetowpreg; heating a selected portion of a workpiece to said melting pointof said thermoplastic resin; applying a first layer of said molten fiberresin composite towpreg to said heated surface of said workpiece wherebya molten state of said molten fiber towpreg is maintained during awrapping procedure thereby resulting in said molten fiber resincomposite towpreg more efficiently adhering to the heated surface of theworkpiece; heating a selected portion of said workpiece having anapplied layer of said fiber resin composite towpreg, wherein saidapplied layer of said fiber resin composite towpreg is melted prior toapplying an additional layer of molten fiber resin composite towpreg;applying an additional layer of molten fiber resin composite towpreg tosaid molten applied layer of fiber resin composite towpreg wherebyapplying said molten fiber resin composite towpreg to an area of moltenfiber resin composite towpreg previously applied to said workpieceallows successive molten fiber resin composite towpreg layers to adhereto one another and further allows individual filaments within saidmolten fiber resin composite towpreg to slide relative to one another,thereby effectively eliminating catenary, wrinkles, and creases withinthe fiber resin composite overwrap; and compacting and consolidatingsaid molten fiber resin composite towpreg layers to one another underpressure thereby removing entrapped air.
 13. The in-situ melt processfor creating a fiber resin composite overwrap of claim 12 wherein saidworkpiece is selected from a group consisting of a Type II pressurevessel, a Type III pressure vessel, a Type IV pressure vessel, and aType V pressure vessel.
 14. The in-situ melt process for creating afiber resin composite overwrap of claim 12 wherein said fiber isselected from a group consisting of carbon fiber, glass fiber, naturalfiber, nano-fiber, and aramid fiber.
 15. The in-situ melt process forcreating a fiber resin composite overwrap of claim 12 wherein saidthermoplastic resin is selected from a group consisting of nylon resin,polypropylene resin, polyethylene resin, and polyetheretherketone resin.16. The in-situ melt process for creating a fiber resin compositeoverwrap of claim 12 wherein said thermoplastic resin is in pellet form.17. The in-situ melt process for creating a fiber resin compositeoverwrap of claim 12 wherein said thermoplastic resin is in tape form.18. The in-situ melt process for creating a fiber resin compositeoverwrap of claim 12 wherein said thermoplastic resin is in thread form.